Neutron tools are used widely in the petrochemical industry, particularly during the so-called LWD (Logging While Drilling) stage or during wireline logging after the well has been drilled. Logging while drilling is performed during the initial stage of drilling a hole down into the earths crust towards an identified hydrocarbon reservoir, which should eventually form a producing oil or gas well for fulfilling our energy needs.
Although various surface techniques exist for characterizing subsurface formations, it is useful to use subsurface techniques for more accurate localized measurements of the surrounding rock formations. This becomes even more useful the deeper the drill progresses. In the case of LWD, the measurement or logging of such data as one progresses down the well, is useful in making more up-to-the-minute estimates as to whether the hydrocarbon reservoir is of sufficient quality to make it economically feasible for production. It also helps in deciding on the optimal location in the hydrocarbon formation to stop the drilling or change its direction and to decide on when to set the casing.
FIG. 1 illustrates a wellsite system in which the present invention can be employed. The wellsite can be onshore or offshore. In this exemplary system, a borehole 11 is formed in subsurface formations by rotary drilling in a manner that is well known. Embodiments of the invention can also use directional drilling, as will be described hereinafter.
A drill string 12 is suspended within the borehole 11 and has a bottom hole assembly 100 which includes a drill bit 105 at its lower end. The surface system includes platform and derrick assembly 10 positioned over the borehole 11, the assembly 10 including a rotary table 16, kelly 17, hook 18 and rotary swivel 19. The drill string 12 is rotated by the rotary table 16, energized by means not shown, which engages the kelly 17 at the upper end of the drill string. The drill string 12 is suspended from a hook 18, attached to a traveling block (also not shown), through the kelly 17 and a rotary swivel 19 which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used.
In the example of this embodiment, the surface system further includes drilling fluid or mud 26 stored in a pit 27 formed at the well site. A pump 29 delivers the drilling fluid 26 to the interior of the drill string 12 via a port in the swivel 19, causing the drilling fluid to flow downwardly through the drill string 12 as indicated by the directional arrow 8. The drilling fluid exits the drill string 12 via ports in the drill bit 105, and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows 9. In this well known manner, the drilling fluid lubricates the drill bit 105 and carries formation cuttings up to the surface as it is returned to the pit 27 for recirculation.
The bottom hole assembly 100 of the illustrated embodiment comprises a logging-while-drilling (LWD) module 120, a measuring-while-drilling (MWD) module 130, a rotary-steerable system and motor 150, and drill bit 105. The LWD module 120 is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at 120A. (References, throughout, to a module at the position of 120 can alternatively mean a module at the position of 120A as well.) The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the present embodiment, the LWD module includes a nuclear measuring device or neutron tool to measure, for example, the porosity of the surrounding formation.
The MWD module 130 is also housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device.
FIG. 2 shows a logging-while-drilling nuclear device as disclosed in U.S. Pat. Re. 36,012, incorporated herein by reference, which utilizes an accelerator-based source, it being understood that other types of nuclear LWD tools can also be utilized as the LWD tool 120 or part of an LWD tool suite 120A. In FIG. 2, a drill collar section 1040 is shown as surrounding a stainless steel tool chassis 1054. Formed in the chassis 1054 to one side of the longitudinal axis thereof (not visible in this view) is a longitudinally extending mud channel for conveying the drilling fluid downward through the drill string. Eccentered to the other side of the chassis 1054 are a neutron accelerator 1058, its associated control and high voltage electronics package 1060 and a coaxially aligned near-spaced detector 1062. The near-spaced detector 1062 is primarily responsive to accelerator output with minimum formation influence. The detector 1062 is surrounded, preferably on all surfaces except that adjacent to the accelerator 1058, by a shield 1064 of combined neutron moderating-neutron absorbing material. The output of the near detector 1062 is used to normalize other detector outputs for source strength fluctuation. Located longitudinally adjacent to the near-spaced detector 1062 is a plurality or array of detectors, of which 1066a and 1066d are shown in this view. The detector 1066a is back-shielded, as shown at 1068a. The array includes at least one, and preferably more than one, epithermal neutron detector and at least one gamma ray detector, represented in this example at 1084, with shield 1086. One or more thermal neutron detectors can also be included. The above-referenced U.S. Pat. Re. 36,012 can be referred to for further details. The detector signals can be utilized to determine, inter alia, formation density, porosity, and lithology.
So-called neutron tools are used for measuring the porosity of the surrounding rock formations and hence, estimating the hydrocarbon (oil or gas) content. The neutron tools have a source responsible for generating neutrons and irradiating the surrounding rock formation with neutrons. Rock formations with a high porosity indicate a fluid such as a hydrocarbon in the formation.
Neutron porosity measurements rely on the fact that hydrogen is an excellent moderator of fast neutrons. The presence of hydrogen (water or oil) in the porespace of rock changes the average distance traveled by neutrons. If the flux of thermal neutrons is measured at an axial distance from the source, it will decrease with increasing hydrogen content and therefore with increasing liquid filled porosity.
Most neutron tools have neutrons arriving from all directions in which there is little or no focusing applied. In this context focusing means limiting the angular and/or vertical extent of the solid angle into which a source can emit radiation or from which a detector can accept radiation. A reason for this is the relatively low power of traditional neutron sources and thus by detecting neutrons from all directions, measurement statistics are enhanced. However, this configuration results in increased sensitivity of the tool to the borehole, i.e. neutrons scattered from the borehole, and not from the formation. This makes the neutron tools unsuited for borehole imaging applications, which have been developed for LWD tools.
The multiple scattering that neutrons typically undergo, not only in the formation but also in the tool before arriving at the detector, reduces the azimuthal sensitivity to the actual formation under investigation.
Some tools have tried to enhance the sensitivity to neutrons from the formation, as compared to the borehole, by eccentering the tool against the formation wall with an eccentering device, typically a bowspring as shown in FIG. 3 for the case of a wireline or slickline tool. It is also possible to position the detectors and in some cases also the neutron source eccentered in the tool to try be more sensitive to neutron scattering deriving from the formation. Another technique is to try back-shield the detectors inside the tool housing to reduce the effect from neutron scattering in the borehole and more importantly the mud channel.
In downhole environments it is often not desirable to have eccentering devices and positioning mechanisms that complicate the tool. Backshielding is not an option on smaller tools, which do not have room behind the detectors to accommodate any shielding.